This invention relates to radio broadcasting, and more particularly, to modulation formats for use in AM In-Band On-Channel (IBOC) Digital Audio Broadcasting (DAB), and transmitters and receivers that utilize such modulation formats.
Digital Audio Broadcasting is a medium for providing digital-quality audio, superior to that provided by existing analog broadcasting formats. AM IBOC DAB can be transmitted in a hybrid format where it coexists with the AM signal, or in an all-digital format where the removal of the analog signal enables improved digital coverage with reduced interference. AM IBOC DAB requires no new spectral allocations because each DAB signal is simultaneously transmitted within the spectral mask of an existing AM channel allocation. IBOC DAB promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners.
U.S. Pat. No. 5,588,022 discloses a hybrid AM IBOC broadcasting modulation format for simultaneously broadcasting analog and digital signals in a standard AM broadcasting channel that includes an amplitude modulated radio frequency signal having a first frequency spectrum, wherein the amplitude modulated radio frequency signal includes a first carrier modulated by an analog program signal, and a plurality of digitally modulated carrier signals within a bandwidth which encompasses the first frequency spectrum, with the digitally modulated carrier signals being modulated by a digital program signal. A first group of the digitally modulated carrier signals lying within the first frequency spectrum is modulated in-quadrature with the first carrier signal, and second and third groups of the digitally modulated carrier signals lying outside of the first frequency spectrum are modulated both in-phase and in-quadrature with the first carrier signal.
U.S. Pat. No. 6,243,424 discloses a method for AM IBOC DAB that uses a center channel signal in a central frequency band of an AM radio channel, wherein the center channel signal includes a carrier which is analog modulated by the first version of the program material and subcarriers modulated by a second version of the program material. Subcarriers in upper and lower sidebands of the AM radio channel are modulated with additional digitally encoded portions of the program material.
The digital signal in AM IBOC DAB systems is modulated using orthogonal frequency division multiplexing (OFDM). OFDM is a parallel modulation scheme in which the data stream modulates a large number of orthogonal subcarriers that are transmitted simultaneously. OFDM is inherently flexible, readily allowing the mapping of logical channels to different groups of subcarriers.
This invention seeks to improve the performance of hybrid AM IBOC DAB systems.